When surveying geological strata, for example, drilling is carried out to investigate the nature and content of the strata. In some such drilling, as in oil drilling, the drill hole is simply directed at the strata to be studied and the drilling waste is removed via the surface.
In other types of investigation it is common at a certain stage in the survey work, to conduct what is known as "core drilling". In this procedure a special form of drill bit is used which drills out a donut-shaped portion of the strata leaving a cylindrical portion of the geological strata, called core, which is then stored within a container or core barrel housed within the drill rods. When a certain length of core has been drilled, the core barrel is then withdrawn and the core is removed and the core barrel is then lowered down the hole again.
In this way, it is possible to obtain a continuous section of core down through a geological strata and thus collect accurate information concerning its composition.
It is well known, however, in both core drilling, and other forms of drilling operations, that the drill bit may deviate from the desired drilling direction and inclination, and the inclination of the bore hole will gradually change. As a result, it is impossible to know the precise point and location in the geological strata to which the information relates. This uncertainty reduces the value of the information obtained.
In order to overcome this, various systems are used to measure the inclination of the bore hole. Readings must be taken at various levels down the hole in order to record its inclination throughout its length. One common method is the use of acid, in a glass test tube, the acid etching the tube and indicating the inclination of the hole. This system is carried out during drilling by lowering the tube down the hole to the desired depth, for each reading. There is a predetermined time delay required for the etching process. During this time drilling must be halted. This is time consuming and causes loss of production.
Other systems using the lowering of an instrument can be used. However, these are costly and result in lost production.
Sometimes this information is used to adjust and control the direction of the drill, when drilling is resumed.
It is also possible with some systems that an instrument may be lowered down the hole after drilling has been completed to thus measure the inclination of the completed hole at selected depths. This is an additional operation, which, while it does not delay drilling itself, does involve further time at the drilling site, and additional expense. In addition, if the hole is not measured until drilling is completed, then it is not possible to use the information to correct the inclination of the drill hole to alter its course.
For accuracy in drilling it is desirable to measure the dip angle frequently, at short intervals. However, as a result of the time delays involved in existing measurement systems it is the usual practice to measure only at intervals of about one hundred feet. Any shorter interval becomes prohibitively expensive.
In the case of core drilling it would be a considerable improvement over this procedure, if an instrument were included in the drill rods at the time the core was being drilled out. The instrument would measure the inclination of the hole so that when the core barrel was removed from the hole, the inclination of the hole at that depth could be recorded. Core barrels vary in length but are commonly between ten and twenty feet. If the measuring instrument gave a reading each time the core barrel was removed, this would greatly increase the frequency of measurement, during drilling. This in turn would result in more accurate information, and would enable the driller to correct the inclination if desired.
One of the problems involved in designing such an instrument is that the drill rods and the core barrel during the drilling procedure rotate at a predetermined speed necessary for drilling through the strata. Thus, any instrument which is incorporated in the drill rods must be capable of withstanding extended periods of relative rapid rotation, and at the end of such movement, being then capable of taking an accurate reading of the inclination of the hole at that level, before the core barrel is removed. In addition, the inclination reading taken must be preserved so as to be capable of being recorded when the core barrel is brought to surface.
Similar problems arise in measuring inclination in other drilling operations, where cores are not obtained. In this case such an instrument would desirably be incorporated in the drill rods, so that readings could be taken each time the rods were removed, without having to lower an instrument down the hole as a separate operation.
In the past, no instrument suitable for this purpose has been available to the industry, and these functions have not been performed.